Addition agent for copper plating



United States Patent ADDITION AGENT FOR COPPER PLATIN G Christian J. Wernlund, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application July 11, 1951, Serial No. 236,270

9 Claims. (Cl. 20452) This invention relates to electroplating copper and more particularly to the electrodeposition of smooth copper deposits.

In the manufacture of automobile bumpers and other chrome plated steel articles, it is desirable to have a smooth, mirror-like chromium finish. In conventional procedure, the steel articles first are mechanically polished with a fine abrasive, then after suitable cleaning they are electroplated with copper and the copper electrodeposit is buffed to a smooth, mirror-like finish. It is then overplated with nickel and if necessary the nickel is also buffed so as to obtain a smooth, mirror-like finish of nickel. A final thin layer of chromium is electroplated onto the nickel plate, producing a smooth, mirror-like chrome finish.

It is possible to produce bright, mirror-like deposits of copper and nickel and such procedures ofier a method of chrome plating steel which would eliminate the bufiing operations. However, when a bright copper deposit is electroplated onto a polished steel article, the tiny scratch marks produced by the abrasive used to polish the steel are visible in the copper coating. In fact, the scratch marks often are accentuated in the copper coating. Each scratch mark is essentially a tiny V-shaped gouge or trough in the steel and there is a definite tendency for the copper to preferentially electrodeposit along the edges of these scratches, which results in an accentuated reproduction of the scratch mark in the bright copper electrodeposit. Unless such images of the scratch marks are removed from the copper plate, e. g., by buffing, the desired smooth, mirror-like chrome plate cannot be obtained by subsequent nickel and chrome plating operations.

A relatively new method of electroplating copper has been devised which is termed current reversal electroplating or periodic current reversal electroplating by which relatively thick copper deposits may be made which are smooth and bright and which tend to eliminate surface imperfections such as pits, scratch marks and the like. However, current reversal electroplating as it has heretofore been practiced has not been suitable for eliminating the scratch marks in bright copper plating in a suitably economical chrome finishing process. While such scratch marks can be eliminated to some extent by conventional current reversal plating, this generally requires building up a relatively thick copper plate, considerably thicker than 0.0003 to 0.0020 inch, which is the thickness range generally used in copper undercoats for chromium plating.

While the various conventional copper plating solutions are suitable for periodic current reversal electroplating operations, the best results generally are obtained by employing a copper cyanide solution. A copper cyanide plating solution which has given excellent results in current reversal plating is described in Wernlund U. S. P. 2,347,448. This bath has a relatively high alkalinity, containing alkali metal hydroxide, and contains alkali metal thiocyanate in concentrations as high as 8 ounces per gallon. When the electroplating bath described in the above Wernlund patent is utilized in a current reversal plating process, for example, by the method disclosed in Jernstedt U. S. P. 2,451,341, exceptionally heavy copper electrodeposits free from nodules and like imperfections may be obtained. However, for plating thin copper deposits such as those commonly used as undercoat for nickel and chrome finishes applied to conventionally police ished steel surfaces such as automobile bumper bars and the like, the polishing marks of the steel are distinctly visible in the copper electrodeposit and the copper deposit generally has to be buffed in order to obtain acceptable surfaces, suitable to serve as undercoat for nickel and chrome plate.

A method for electroplating exceptionally smooth and bright copper electrodeposits is disclosed in the copending application for Letters Patent by Henry G. McLeod and Donald A. Swalheim, Serial No. 174,572 filed July 18, 1950, now abandoned. That method is characterized chiefly by current reversal electrolysis of a copper cyanide solution containing a small amount of a water solube selenate or selenite and substantially free from thiocyanate and selenocyanate ions. While that process produces excellent bright copper electrodeposits over a fairly wide range of current density, generally the production of electrodeposits sufficiently smooth and bright to serve as an undercoat for chromium plating or nickel and chromium plating without bufiing the copper deposit is restricted to a current density range of about 30 to 60 amperes per square foot.

An object of this invention is to improve the process of the above-mentioned McLeod and Swalheim application whereby the current density range over which excellent smooth and bright copper electrodeposits are obtained is substantially broadened.

A further object is to provide a new and improved current reversal electroplating process for electroplating copper, including a new addition agent for copper cyanide plating baths. A further object is to provide a new and useful copper cyanide plating solution. Another object is to provide an improved method for producing smooth, mirror-like, chromium electrodeposits on base metals. A further object is to produce on steel surfaces copper deposits having a mirror-like finish, free from scratch marks, nodules, burning and other surface imperfections. Still other objects will be apparent from the following description of our invention.

The above objects are attained in accordance with the present invention by current reversal electroplating of copper from a copper cyanide solution which is substantially free from thiocyanate and which contains a water soluble selenate or a water soluble selenite in concentration equivalent to 5 to 500 parts per million of elemental selenium, and about 0.025 to about 50 grams per liter of a water soluble salt of methylene-bis-(a-napththalene sulfonic acid). The periodic reversal of the electroplating current is so conducted that the cathodic time, i. e., the time during which the article being plated as cathode, is not less than 15 seconds and preferably is 30 to seconds. The anodic current time during each cycle of current alternation is less than the cathodic time but in any case will be not less than 2 seconds. In any case, the total amount of current flowing while the article to be plated serves as cathode should exceed by at least 10% the amount of current flowing while said article serves as anode. If desired, the applied current density may be different during the cathodic and anodic times, provided that the coulombs of applied cathodic current is at least 10% greater than the coulombs of the applied anodic current. Generally there is little or no advantage in varying the current densitites with current alternation; and it is preferred to maintain a constant applied current density while alternating the direction of current flow.

If the selenium compound is eliminated in the above process, the presence of the above-mentioned sulfonic acid addition agent produces little or no brightening eflect. On the other hand, the addition of the sulfonic acid compound markedly increases the current density range over which bright electrodeposits of high quality are produced in copper cyanide plating baths operated according to the current reversal technique and containing the above-mentioned selenium compound.

The following example serves to illustrate the invention.

Example Copper was electrodeposited from aqueous plating solutions containing copper potassium cyanide, made alkaline by addition of caustic soda and sodium carbonate and containing a small amount of sodium selenate meeting to the following analysis:

Ingredient Concentration 7 to 8 oz./gal.

1.2 to 1.4 ozJgal. 3 to 4 oz./gal. 3 ozJgal. 50 parts per million.

Polished sheet steel panels were plated from the above electrolyte in an oscillating cathode Hull cell, in which the current density at the steel suface increased across the panel in a horizontal direction. The equivalent of solution agitation equal to about 24 feet per minute was obtained by reciprocally moving the steel panel in a vertical direction. The plating current was periodically reversed at a cycle of 60 seconds cathodic time and i8 seconds anodic time. The average current density at the steel panel was about 40 amps/sq. ft. The panel was plated for a period of 30 minutes, to obtain a copper deposit varying from about 0.002 to 0.0004 inch thick.

The character of the surface of the steel panel and of the electrodeposit was measured by a conventional brush surface analyzer, the average surface roughness or R. M. S. (root mean square) value being expressed in microinches. On the electrodeposit, the R. M. S. value was determined at locations, evenly spaced horizontally across the panel from the low current density side to the high current density side. The panel was 4.5 inches wide and the locations of R. M. S. measurement were spaced 0.5 inch apart.

The foregoing procedure was repeated, except that the plating solution additionally contained 300 parts per million of methylene-bis-(oz-naphthalene sulfonic acid). The following results were obtained. In the table, A designates the original plating solution, while B designates the solution containing the methylene-bis-(wnaphthalene Locations 1 and 8 at the high and low current density sides, respectively.

The electrodeposit on panel A was mirror-bright only on the high current density area, i. e., from location 1 to about midway between locations 4 and 5. On panel B, the electrodeposit was mirror-bright over the entire range from location 1 to location 8.

It is essential that the cyanide plating bath utilized in practicing this invention be substantially free from soluble thiocyanate, as the presence of thiocyanate ions inhibits or prevents the smoothing or leveling action of the selenium compound addition agent. While a small amount of thiocyanate, such as 25 to 50 parts per million, has little detrimental effect, amounts higher than 200 parts per million are distinctly detrimental and should be avoided. The bath also must be free from selenocyanide ions (CNSe), as a concentration as low as 0.25 gram per liter of alkali metal selenocyanide has been found to inhibit leveling. current time in each cycle of current alternation be of at least 15 seconds duration and preferably of at least seconds duration. This is because of a phenomenon which occurs in the plating bath of this invention, which has not heretofore been reported in current reversal plating operations. In the conventional current reversal plating processes, utilizing a copper cyanide plating solution which tends to give bright copper deposits, generally little or no change in the appearance of the electrodeposit can be observed as the direction of the current changes. In operating current reversal in accordance with the present invention, the observed phenomena are entirely different. During the latter part of the cathodic portion of the current reversal cycle, the plate has a bright polished appearance. When the current is reversed (made anodic),

It is also essential that the cathodic h the electrodeposit slowly becomes darker in appearance and at the end of, e. g. 3 to '10 seconds or more, becomes dull in appearance. When the current then is again reversed to make the article being plated cathodic, the color of the electrodeposit is suddenly changed to a deep red, dull color characteristic of burnt copper electrodeposits. This red color gradually fades and in a period of time, which may vary from around 15 to 40 seconds, the red color is entirely dissipated leaving a clear, bright copper electrodeposit which, by contrast with the red color, as seen through the transparent side walls of an electroplating cell constructed of Lucite (methacrylate resin), appears almost like highly polished brass. For best results, it is desirable to continue the cathodic current at least until all of the red color has disappeared, before again reversing the current. In most cases this will require 30 to 40 seconds, although in some instances the time required to dissipate the red color and brighten the plate may be as low as 15 seconds.

After the cathodic current has been continued sufficiently to dissipate the red color, it may be further continued for as long as a total of 150 seconds before again reversing the current. If the cathodic current time is longer than about 150 seconds, roughness tends to develop in the electrodeposit. A cathodic current time of 30 to seconds generally yields the best results.

The duration of the anodic current time is not critical, so long as the amount of current flowing during the anodic current time is less than that flowing during the cathodic time, provided that the anodic current time in any case is not less than about 2 seconds. The best results are generally obtained when the net current efiiciency is within the range of 20 to 80%. Net current eificiency as used herein means the figure obtained by dividing the total amount of current passed through the electroplating cell into the difference between the amount of current passed through while the article to be plated is cathode and the amount of current passed through while the article is the anode. This figure should be multiplied by if it is desired to express the net current efficiency as a percentage. An equation for the current efiiciency may, therefore, be written as:

where N =net current etliciency Tc=cathodic current time Ta=AI10dlC current time Dc=cathodic current density Da=anodic current density At constant current density (Dc=Da), the equation becomes:

Thus, when the current reversal cycle is 50 seconds cathodic and 15 seconds anodic at constant current density, the net current efficiency equals:

Agitation of the electrolyte is also essential to obtain an appreciable leveling effect. The various methods for agitating electroplating solutions as commonly practiced or known in the art may be utilized. Such methods include stirring the electroplating bath by means of agitators or stirring devices installed in the electroplating cell or by passing a stream of air or other gas into the electrolyte, flowing a stream of electrolyte across the face of the surface to be electroplated, moving the article to be electroplated, either in reciprocal or continuous linear motion or by rotation, or combinations of such means so as to effect a relative movement of the electrolyte with respect to the surface being electroplated. While agitation is essential in carrying out this invention, the degree of agitation may be varied over an exceedingly wide range, provided that it is equivalent to a linear flow of electrolyte across the surface to be plated of at least 10 feet per minute. Much higher rates of flow may be utilized if desired, including flow rates as high as 1,000 feet per minute or even higher. However, such high flow rates generally are no more advantageous than slower rates, up

to around 250 feet per minute. Generally it is preferred to operate this electroplating bath with agitation equivalent to an electrolyte flow rate across the surface of 20 to 100 feet per minute.

The herein described process may be operated over a wide range of current densities, as in conventional copper plating practice. Generally, we prefer to operate at a current density of about 30 to 60 amperes per square foot. The primary factor determining the optimum current density is the degree of agitation of the electrolyte. For example, with agitation of around 25 feet per minute, the best results are obtained at a current density of about 40 amperes per square foot, while at agitation of 50 to 100 feet per minute, higher current density is preferred. The optimum current density is also affected by factors such as the copper content and free cyanide concentration in the bath, substantially as such factors affect optimum current density in conventional cyanide copper plating processes.

Herein, and in the appended claims current and current density refers to the current or current density applied to the article being electroplated. The current density at the other electrode may vary in accordance with conventional practice in copper electroplating.

While any copper cyanide plating bath which is free from thiocyanate may be utilized in practicing this invention, it is preferred to use a bath which contains a considerable proportion of alkali, that is, alkali equivalent to l to 8 ounces per gallon of sodium hydroxide. The alkali metal cyanide used in the plating bath may be either sodium cyanide or potassium cyanide or the bath may contain both sodium and potassium ions in any desired ratio. I generally prefer to use potassium cyanide, and usually the best results are obtained with electrolytes containing a molar ratio of potassium ions to sodium ions not less than 1. A preferred amount of alkali metal cyanide is that equivalent to from 1 to 2 ounces per gallon of sodium cyanide in excess of the amount required to form the copper alkali metal double cyanide, e. g., NazCu(CN)3 or KzCu(CN)3. Such excess of cyanide is herein referred to as free cyanide.

In practice, baths initially made up to contain copper cyanide, alkali metal cyanide and alkali metal hydroxide, after a period of operation generally contain considerable quantities of carbonate due to absorption of carbon dioxide from the air. Therefore, if desired, the bath initially may be made to contain both alkali metal hydroxide and carbonate. Also, as in conventional cyanide copper plating operations, during prolonged periods of operation a small but appreciable amount of ferrocyanide will be formed in the bath. We have found that the presence of the ferrocyanide has no appreciable effect on the results obtained.

The preferred range of solution ingredients in such ailililnfl copper cyanide baths is shown by the following ta e:

Copper alkali metal double cyanide equivalent: CuCN,

7 to 12 oz./gal. (52.5-90 g./l.)

Free alkali metal cyanide equivalent: NaCN, l to 2 oz./gal. (7.5-15 g./l.)

Alkali metal hydroxide equivalent: NaOH, 1 to 8 oz./ gal.

Alkali metal carbonate equivalent: NazCOg, 2 to 6 oz./gal. (15-45 g./l.)

The preferred electrolyte temperature is in the neighborhood of 80 C., e. g., 75 to 85 C. However, bath temperature is not critical and may follow conventional practice, i. e., 70 to 90 C.

The selenium compounds used as addition agents in accordance with the present invention are the water soluble selenites and selenates. The preferred selenium compounds are the water soluble alkali metal selenites and selenates. As all water soluble selenites and selenates are equally effective as addition agents, it is evident that any compound which when added to the bath will produce in solution therein the selenite ions -HSeO3 or =SeO3, or the selenate ions HSeO4 or =SeO4 may be employed. The amount of the selenium compound may be varied over a wide range and exceedingly small amounts will have a noticeable effect on the smoothness of the electrodeposit. Amounts of selenite or selenate as low as that equivalent to parts per million (0.005 g./l.) of elemental selenium (Se) are effective. If desired, amounts up to about 0.5 gram per liter (500 p. p. m.) may be utilized. At concentrations substantially above 0.5 gram per liter the desired brightening and leveling effects are not obtained. The selenium compound, or its effectiveness, gradually disappears during both operations and must be replenished by periodic addition. Generally replenishment at a rate equivalent to around 5 parts per million of elemental selenium per ampere-day of operation will suffice.

The organic addition agent of this invention is methylene-bis-(ct-naphthalene sulfonic acid), which is an article of commerce and may be prepared, for example, by reacting formaldehyde with a-naphthalene sulfonic acid.

The best results are generally obtained by maintaining the methylene-bis-(zit-naphthalene sulfonic acid) in the electrolyte in a concentration of from 0.5 to 5 grams per liter. However, this addition agent is very effective over a wide range of concentration including from 0.025 to 50 grams per liter. In practical operation a suitable concentration of the organic addition agent may be maintained by adding 0.2 to 0.5 gram per liter every 3 to 6 days or whenever low current density areas of the work do not exhibit the desired degree of smoothness and brightness.

The methylene-bis-(rt-naphthalene sulfonic acid) may be added to the electroplating bath either as the free acid or as an alkali metal salt, for example, the sodium or potassium salt. Generally, it is preferred to add this addition agent in the form of an aqueous solution of an alkali metal salt. When the free acid is added to the plating bath it reacts with the alkali present to form the water soluble alkali metal salt, which apparently is the active addition agent.

Electrolyte compositions suitable for practicing my invention comprise not only the aqueous copper cyanide solutions employed as plating baths, but likewise concentrated or solid plating salt compositions free from soluble thiocyanate or selenocyanide compounds, containing copper cyanide and a suitable quantity of a water soluble selenite or selenate and methylene-bis-(a-naphthalene sulfonic acid) or a water soluble salt thereof. The proportion of the selenate or selenite and of the methylene-bis-(at-naphthalene sulfonic acid) or its alkali metal salt in such solid or concentrated compositions will depend upon the copper cyanide content, so that when the composition is added to water, together with any other necessary or desired plating bath ingredients to form a solution suitable for use as a plating bath, the solution will contain the selenite or selenate in concentration equivalent to 0.005 to 0.5 gram per liter of elemental selenium and 0.025 to 50 grams per liter of the sulfonic acid. It will be seen that the range of effective seleniumzsulfonic acid ratios is very broad. It may, in fact, be represented as 0.005-0.510.025-50, the relative weights of the respective ingredients varying within the limits indicated. These ratios apply of course to both the solution and the solid mixture. Such solid or concentrated compositions are convenient for the plater to use, for bath replenishment as well as for initial makeup, as they insure that the bath will always contain a proper amount of the selenite or selenate. Preferably, such compositions are anhydrous, but if desired they may be concentrated aqueous solutions or slurries.

The simplest of such solid or concentrated compositions is a mixture of copper cyanide (CuCN) with the selenate or selenite and the sulfonic acid or its alkali metal salt. In making up the plating bath, this mixture is added to water together with sufficient alkali metal cyanide to form the soluble double cyanide, and preferably with suitable addition of alkali such as alkali metal hydroxide. A preferred composition is a mixture of the selenite or selenate with a water soluble copper double cyanide such as Na2Cu(CN)3 or KzCu(CN)3. Such mixtures may conveniently be made by mixing together the copper salt with either the dry, powdered or crystalline selenite or selenate or with an aqueous solution thereof. The methylene-bis-(x-naphthalene sulfonic acid) or its salt may be added to the mixture in dry, solid form or as an aqueous solution. For example, to make a homogeneous mixture, granular of powdered copper cyanide or the double cyanide may be mixed with aqueous solutions of the selenium compound and the sulfonic acid, so that each particle is wet with the solutions. The composition then may be dried or not, before packaging for shipment. Various other methods of making such mixtures will be apparent to those skilled anonaaa in making plating salt compositions. If the entire contents of a package of the plating composition is to be dissolved to make a plating solution, the mixture of the copper salt, selenium compound and sulfonic acid need not be homogeneous. Since the equivalent CuCN concentration in the final electroplating solution is about 52.5-90 g./l., the weight ratio of cuprous cyanide:elemental selenium:sulfonic acid in that solution and hence in the dry salt mixture from which it is made may be represented as 52.590:0.005-0.5:0.25-50.

The herein described electroplating process is useful for producing smooth, bright electrodeposits on the surface of any structural metal. Its greatest utility however appears to reside in the electroplating of steel articles with copper to produce smooth mirror-bright copper electrodeposits which may be utilized without bufiing or other treatment as undercoats for nickel and chromium plating. Steel articles such as automobile bumper bars which have been polished by conventional methods using polishing grits of No. 180 to No. 300 can be electroplated with copper by the herein described method to a thickness of not more than 0.002 inch, to form a smooth mirror-bright electrodeposit in which few or none of the polishing marks on the steel are visible. Without buffing or other mechanical treatment, the copper electrodeposit may be utilized in conventional manner to serve as undercoat for nickel and chrome plates to produce finishes of high quality. The present invention is likewise useful for copper plating non-ferrous metal articles such as die castings and the like, for nickel and chrome finishing of such non-ferrous articles.

The presence of the methylene-bis-(ti-naphthalene sulfonic acid) in the electrolyte causes the above results to be obtained over a very wide range of current density, generally from 15 to 60 amperes per square foot. This insures a uniformly smooth and bright surface on all parts of irregularly shaped articles.

While the utility of the invention is primarily to copper plate steel surfaces, the invention is not restricted thereto; but it may be utilized to copper plate other base metals or conductive bodies with comparable results.

I claim:

1. The process for electroplating copper which comprises electrodepositing copper from a copper cyanide plating solution which is substantially free from thiocyanate and selenocyanide ions and which contains a selenium compound selected from the group consisting of the water soluble selenates and selenites, in concentration equivalent to 0.005 to 0.5 grams per liter of elemental selenium and 0.025 to 50 grams per liter of, an alkali metal salt of methylene-bis-(ct-naphthalene sulfonic acid), while periodically reversing the electroplating current in such manner that in each cycle of current alternation the cathodic current time is not less than about 15 seconds nor more than about 150 seconds and the anodic current time and current density is such that the cathodic current exceeds the anodic current by at least 10% and the anodic current time is at least 2 seconds, with solution agitation equivalent to a flow of elec trolyte across the cathode surface at a velocity of about 10 to 1,000 feet per minute.

2. The process for electroplating copper which comprises electrodepositing copper onto a steel surface from a copper cyanide plating solution which is substantially free from thiocyanate and selenocyanide ions and which contains a selenium compound selected from the group consisting of the water soluble selenates and selenites, in concentration equivalent to 0.005 to 0.05 g./l. of elemental selenium, and 0.5 to 5 grams per liter of an alkali metal salt of methylene-bis-(at-naphthalene sultonic acid) while periodically reversing the electroplating current in such manner that in each cycle of current alternation the cathodic current time is from 30 to 90 seconds and the anodic current time and current density is such that the net current efiiciency lies within the range of about 20 to and the anodic current time is at least 2 seconds, while maintaining the cathodic and anodic current densities within the range of about 30 to 60 amperes per square foot, with solution agitation equivalent to a flow of electrolyte across the cathode surface at a velocity of about 20 to 100 feet perminute.

3. The process according to claim 2 in which the cur rent density is maintained substantially constant, within the range of about 30 to 60 amperes per square foot and the electrolyte is a solution of copper-alkali metal double cyanide containing from 1 to 2 oz. per gallon of free alkali cyanide and has an alkalinity equivalent to 1 to 8 oz. per gallon of sodium hydroxide.

4. The process according to claim 3 in which said electrolyte corresponds to the following formula:

Copper cyanide: CuCN equivalent, 7 to 12 oz./ gal.

Free cyanide: NaCN equivalent, 1 to 2 oz./ gal.

Alkali metal hydroxide: NaOH equivalent, 1 to 8 oz./ gal.

Alkali nlietal carbonate: NazCOa equivalent, 2 to 6 oz. ga

5. A copper plating composition comprising a copper cyanide, a selenium compound selected from the group consisting of the water soluble selenites and selenates and methylene-bis-(oz-naphthalene sulfonic acid) in the weight ratio cuprous cyanidezelemental seleniumzsulfonic acid of 52.5:0.0050.520.02540, said composition being substantially free from materials yielding thiocyanate and selenocyanide ions on solution.

6. The composition of claim 5 in which the copper cyanide is a water soluble, copper-alkali metal double cyanide.

7. The composition of claim 6 in which the copper cyanide is potassium copper cyanide, K2Cu(CN)3.

8. The composition of claim 6 in which the copper cyanide is sodium copper cyanide, NazCu(CN)3.

9. The process of producing bright level copper which comprises electrodepositing said copper from an aqueous plating solution substantially free from thiocyanate and selenocyanide ions and containing about 52.5-90 grams per liter of cuprous cyanide, a selenium compound selected from the group consisting of water soluble selenates and selenites, in concentration equivalent to 0.005 to 0.5 grams per liter of elemental selenium, and about 0.025 to 50 grams per liter of an alkali metal salt of methylene-bis-(ct-naphthalene sulfonic acid) while periodically reversing the electroplating current in such a manner that the cathodic time of the plating cycle is longer than the anodic time and the total cathodic current exceeds the total anodic current.

References Cited in the file of this patent UNITED STATES PATENTS 1,818,229 Lutz et a1 Aug. 11, 1931 2,195,409 Flett Apr. 2, 1940 2,451,341 Jernstedt Oct. 12, 1948 2,563,360 Phillips et al Aug. 7, 1951 OTHER REFERENCES Serial No. 351,241, Weiner (A. P. C.), published May 18, 1943. 

1. THE PROCESS FOR ELECTROPLATING COPPER WHICH COMPRISES ELECTRODEPOSITING COPPER FROM A COPPER CYANIDE PLATING SOLUTION WHICH IS SUBSTANTIALLY FREE FROM THIOCYANATE AND SELENOCYANIDE IONS AND WHICH CONTAINS A SELENIUM COMPOUND SELECTED FROM THE GROUP CONSISTING OF THE WATER SOLUBLE SELENATES AND SELENITES, IN CONCENTRATION EQUIVALENT TO 0.005 TO 0.5 GRAMS PER LITER OF ELEMENTAL SELENIUM AND 0.025 TO 50 GRAMS PER LITER OF AN ALKALI METAL SALT OF METHYLENE-BIS-(A NAPHTHALENE SULFONIC ACID), WHILE PERIODICALLY REVERSING THE ELECTROPLATING CURRENT IN SUCH MANNER THAT IN EACH CYCLE OF CURRENT ALTERNATION THE CATHODIC CURRENT TIME IS NOT LESS THAN ABOUT 15 SECONDS NOR MORE THAN ABOUT 150 SECOND AND THE ANODIC CURRENT TIME AND CURRENT DENSITY IN SUCH THAT THE CATHODIC CURRENT EXCEEDS THE ANODIC CURRENT BY AT LEAST 10% AND THE ANODIC CURRENT TIME IS AT LEAST 2 SECONDS, WITH SOLUTION AGITATION EQUIVALENT TO A FLOW OF ELECTROLYTE ACROSS THE CATHODE SURFACE AT A VELOCITY OF ABOUT 10 TO 1,000 FEET PER MINUTE. 